1. Field of the Disclosure
The present disclosure is directed to an apparatus and method of sintering coatings onto substrates, and includes the use of a flame with an electric field plasma. The method is capable of being used in an open atmospheric environment. An electrical voltage is used to generate an electric plasma produced through the flame, resulting in a combined energy profile sufficient for powder-powder sintering and powder-substrate bonding. This method is referred to as “flame-assisted flash sintering” (FAFS).
2. Background of the Disclosure
Ceramic coatings on metallic substrates serve myriad purposes in a number of applications because the ceramics provide desirable wear, hardness, chemical, appearance, wetting, thermal, or electrical properties. One very important area of use is for heat exchangers, in which the ceramic coating typically serves to shield the underlying metal from unwanted effects due to extreme heat or chemical corrosion. A pure metal is desirable for the most efficient exchange of heat, but modern air-conditioning and energy recovery systems can generate temperatures in excess of 500° C. that can lead to decreased performance and longevity, because of corrosion and oxidation of the metal. Because ceramic materials generally have superior temperature and corrosion resistance, compared with metals, ceramics can extend the life of heat exchangers operating in extreme environments, albeit with some reduction in operating efficiency.
Ceramic coatings are also essential to performance and longevity in thermal barrier coatings (TBC) for gas-turbine engines, among other applications. The hot gas streams in gas-turbine engines can reach temperatures well in excess of 1000° C. and a barrier coating is thus necessary to protect the underlying metal from corrosion and, for TBC, thermally insulating coatings are helpful.
Numerous other applications are known to benefit from ceramic coatings onto metals, including fuel cells, battery-electrode coatings, wire-insulation coatings, wear and abrasion surfaces, cookware, engines, exhaust shields, power plants of various types, biomedical implants, and aerospace applications.
Two common methods to deposit ceramics onto metals are air plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD). In APS, ceramic powder is injected into an acetylene-oxygen flame nozzle that contains a plasma arc formed by a voltage and the high temperatures generated from the combustion process. As the powder feedstock is injected through this hot region (>2500° C.), the powder melts and some consolidates into large droplets that are then conveyed to the metal substrate where they splat-impact, cool, and resolidify. This method is widely used to make thick porous films of ceramics, but is not suitable for making very thin, smooth films with high density and low porosity.
The porosity and smoothness issue is improved when using EB-PVD, where an intense beam of electrons melts and vaporizes a solid ceramic target inside a vacuum chamber. As a melt is formed, vapor-phase material is generated within the low-pressure chamber and a uniform coating is deposited on a nearby substrate. Although this process deposits films that are generally superior to APS, the method is costly, because it is slower and requires expensive vacuum chambers, source targets, and power supplies for beam generation and steering.
Moreover, in any vapor-phase deposition, a large percentage of the target material becomes wasted and deposited on the surrounding chamber walls and the substrate must be manipulated in the vacuum chamber to coat all the surfaces. Thus, cost is a limiting issue with EB-PVD and it is only used in the most demanding applications. Plasma-enhanced chemical vapor deposition (PECVD) is a similar technique in that it is a low-pressure vapor deposition process, but suffers from some of the same cost issues as EB-PVD.
Various techniques exist that use electric fields to sinter ceramic materials. Such techniques are collectively referred to as “field-assisted sintering” (FAST), and include spark plasma sintering (SPS), pulsed electric current sintering (PECS), and flash sintering. In all of these methods, an electric field is applied across a green body material and resistive heating caused by current flow consolidates the powder material. Traditional SPS applies uniaxial pressure to a ceramic green body sample that is sandwiched between two conductive graphite dies that generate the electric field. Commercial versions of such systems exist, but they are not well-suited to handle large-area thin films or complex shapes, and typically require a vacuum atmosphere. Published information shows such electric field-induced sintering has been applied to ceramic parts but not to coatings of ceramic on metals or other conductive substrates.
In a variation on SPS, several publications have demonstrated that so-called “flash sintering” can be used to consolidate ceramics at moderately low temperatures without the need for external pressure or a vacuum. Flash sintering uses an external heating source to bring the ambient temperature of the ceramic to a baseline temperature (for example, as low as ˜850-1000° C. for YSZ), and an electrical current flowing through the sample then consolidates the powder in a matter of seconds. Reduced sintering temperatures and times present a major opportunity for cost savings in materials processing. The actual temperature at which sintering occurs, and the speed of sintering was shown to be controlled by the electric field strength. In each of the above-mentioned field-assisted processes, the physical restriction of having two conductive electrodes limits the geometries of the ceramic parts being sintered.
Although common applications of ceramic coating may be satisfied by the various ceramic coating processes described, there is a continuing need for a method of ceramic coating that produces very little waste in terms of coating material, that works well for large or contoured parts, and that can be applied under atmospheric conditions, free of the burdens of traditional vacuum chambers.